1. Field of the Invention
The disclosure relates to a system and method for a deep draft semi-submersible floating structure for drilling and production. More particularly, the disclosure relates to a system and method for a semi-submersible floating structure to minimize vortex-induced motion.
2. Description of the Related Art
Most conventional semi-submersible offshore platforms for offshore drilling and production comprise a hull that has sufficient buoyancy to support a work platform above the water surface. The hull typically includes at least two horizontal pontoons that support at least three vertical columns which support the deck platform above the surface of the water. Semi-submersible platforms have become a favorable choice as a wet-tree floater support in harsh environments using steel catenary risers (SCR) extending to the seabed, mainly due its capability of quayside topside integration, cost-effectiveness, and acceptable motion when deployed offshore.
FIG. 1 is a perspective schematic diagram illustrating a conventional semi-submersible floating offshore platform design, showing only the underwater part of the hull. A conventional semi-submersible floating offshore platform 1 is deployed in a body of water in deep draft operational configuration and anchored to a seabed by mooring lines (not illustrated). The offshore platform 1 includes generally at least three, and often four, columns 2, spaced apart from each other and extending vertically from the platform base 3. The base is formed, in this example, with at least three, and often four, pontoons 4 coupled to the bottoms 2A of the columns 2. Each pontoon 4 extends between two bottoms of the columns. An exemplary draft of each column 2 is about 20-25 meters (m) for shallow draft platforms and about 35-45 m for deep draft platforms. The offshore platform 1 is generally moored to the seafloor (not shown) by catenary mooring lines 30 extending through fair leads 31 coupled at the lower ends of the columns.
A conventional semi-submersible, for example with a draft of 20 m, has a Vortex-Induced-Motion (VIM) that is acceptably small due to the small VIM excitation from the shallow draft. Vortex-Induced-Motion (VIM) or Vortex-Induced Vibrations (VIV) are motions induced on bodies facing an external flow by periodical irregularities of this flow. Typically, the term VIM is applied to a moored floating structure and the term VIV is applied to SCRs and other risers. Fluids present some viscosity, and fluid flow around a body, such as a cylinder in water, will be slowed down while in contact with its surface, forming a boundary layer. At some point, this boundary layer can separate from the body. Vortices are then formed, changing the pressure distribution along the surface. When the vortices are not formed symmetrically around the body with respect to its midplane, different lift forces develop on each side of the body, thus leading to motion transverse to the flow. VIM and VIV are important sources of fatigue damage of offshore oil exploration and production platforms, risers, and other structures. These structures experience both current flow and top-end vessel motions, which give rise to the flow-structure relative motion. The relative motion can cause VIM/VIV “lock-in”. “Lock-in” occurs when the reduced velocity, Ur, is in a critical range depending on flow conditions and can be represented according to the formula below:5<Ur=uTn/D<7                Ur: Reduced velocity based on natural period of the moored floating structure        u: Velocity of fluid currents (meters per second)        Tn: Natural period of the floating structure in calm water without current (seconds)        D: Diameter or width of column(meters)        
Lock-in can occur when the vortex shedding frequency becomes close to a natural frequency of vibration of the structure. When lock-in occurs, large and damaging vibrations can result.
It is known that deep draft semi-submersibles suffer from VIM due to the increased excitation length of longer columns compared to shallow draft semi-submersibles with shorter columns.
Thus, there remains a need for improved performance with semi-submersible floating structures, particularly deep draft semi-submersible floating structures, regarding VIM.